Method of preventing the polymerization of unsaturated carboxylic acid

ABSTRACT

A method of preventing polymerization of an unsaturated carboxylic acid selected from the group consisting of acrylic acid and methacrylic acid, which comprises adding to the acid, as the polymerization inhibitor, the combination of (i) at least one phosphorus compound selected from the group consisting of phosphoric acid compounds and phosphorus oxide compounds, and (ii) at least one hydroxyl group containing compound selected from the group consisting of cresols, 4,4-thiobis-6-tert.-butyl3-methylphenol, dihydroxybenzene derivatives and pyrogallol, in the amount sufficient to inhibit polymerization of the monomer.

United States Patent 1191 1111 3,818,079

Sat et al. [4 June 18, 1974 [5 METHOD OF PREVENTING THE 2,758,072 8/1956 Sutherland et al 203/60 POLYMERIZATION 0F UNSATURATED 2,978,501 4/1961 Adams 260/526 N CARBOXYLIC ACID 3,462,484 8/1969 Schnizer et al. 203/8 3,639,466 2/1972 Leichtle 260/526 N [75] Inventors: Ryozi Sato, Takaoka; Yasuyoshi 3,658,895 4/1972 Riemann el al.... 203/38 Chino, Tokyo; Tatus Endo, Yokohama, all of Japan Primary Examiner-Wilbur L. Bascomb, Jr. [73] Assignee: The Japanese Geo Company Ltd Attorney, Agent, or FirmSherman & Shalloway Tokyo, Japan 22 Filed: July 2, 1971 [57] ABSTRACT A method of preventing polymerization of an unsatu [21] Appl' 159553 rated carboxylic acid selected from the group consisting of acrylic acid and methacrylic acid, which com- Foreign Application Priority Data prises adding to the acid, as the polymerization inhibi- July 7, 1970 Japan -58764 the Combination of (i) at least one Phosphorus compound selected from the groupconsisting of phos- 52 us. (:1 260/526 N, 203/8, 203/35 Phoric acid compounds and phosphorus oxide 51 1m. (:1. C07c 51/50, CO7c 57/04 pounds, and (ii) t least one y y group contain- [58] Field of Search 203/8, 9, 35, 60, 57, 65; ing Compound selected from the group Consisting of 2 0 52 N cresols, 4,4-thiobis-6-tert.-butyl-3-methylphenol, dihydroxybenzene derivatives and pyrogallol, in the 5 References Cited amount sufficient to inhibit polymerization of the UNITED STATES PATENTS 2,|7l,795 9/1939 Kautter 203/60 12 Claims, No Drawings METHOD OF PREVENTING THE POLYMERIZATION F UNSATURATED CARBOXYLIC ACID This invention relates to stabilization of unsaturated carboxylic acids.

Unsaturated carboxylic acids such as acrylic and methacrylic acids (which may be hereinafter referred to simply as the monomer) are valuable as starting materials in chemical industries. It is well known that they are extremely unstable compounds which are likely to give rise to the polymerization reaction with ease. It is essential, therefore, that such monomers should be sufficiently stabilized (polymerization inhibition of monomers) in carrying out production, storage and transpotation of these monomers. Light and heat are particularly effective agents to induce polymerization of such monomers. Consequently, when the monomers are exposed to high temperatures (50C. and above, particularly 70 150C., and still higher temperatures) in the course of their processing such as distillation and refining, the polymers thereof are precipitated in the system and frequently clog the distillation column, pipes, etc., seriously interfering with the operation. Thus, their strong tendency to be polymerized proves highly disadvantageous industrially and economically.

As the widely used conventional polymerization inhibitor of acrylic acid or methacrylic acid, such compounds as hydroquinone, phenol, t-butylcatechol, cresol, phenothiazine, ethylene thiourea, etc. may be named, but satisfactory results are not necessarily achieved with such known compounds. Particularly many of them fail to stabilize the monomers against high temperatures. Again, while the monomers are especially apt to be polymerized when they are in contact with stainless steel or carbon steel, no sufficiently effective polymerization inhibitor is known in such a case. Furthermore, even those polymerization inhibitors which show some stabilizing effect at high temperatures for short duration seldom exhibit effective stabilization action for prolonged periods.

Accordingly, the object of the present invention is to provide a method for preventing polymerization of acrylic or methacrylic acid, particularly at elevated temperatures, using a novel polymerization inhibitor which can exhibit a sufficient stabilization effect for acrylic or methacrylic acid which are being processed at elevated temperatures.

Other objects and advantages of the invention will become apparent from reading the following descriptron.

As the result of research to find a novel stabilization method meeting the above objects of the invention, we

discovered that the conjoint use of a specific phosphorus compound with a specific hydroxyl groupcontaining compound as the polymerization inhibitor exhibits outstanding effects as a stabilizer for monomers. Accordingly, this invention provides a method of preventing polymerization of an unsaturated carboxylic acid selected from the group consisting of acrylic acid and methacrylic acid, which comprises adding to such acid, as the polymerizations inhibitor, the combination of (i) at least one phosphorus compound selected from I the group consisting of phosphoric acid compounds and phosphorus oxide compounds, and (ii) at least one hydroxyl group-containing compound selected from the group consisting of cresols, 4,4-thiobis-6-tert. butyl-3-methylphenol, dihydroxybenzene derivatives and pyrogallol, in the amount sufficient to inhibit polymerization of the monomer.

The precise mechanism of the stabilization action of the above phosphorus compounds (i) and hydroxyl group-containing compounds (ii) on the unsaturated carboxylic acid is not yet clear. In view of the fact, however, that in the control runs wherein (a) phosphorus compounds outside the specified phosphorus compound (i) were used with the compound (ii), (b) hydroxyl group-containing compounds outside the scope of compound (ii) were used with the compound (i), (c) the compound (i) alone was used, and (d) the compound (ii) alone was used, the stabilization effect was observed in no case, and that the conjoint use of the compounds (i) and (ii) exhibits absolutely no synegistic effect in stabilization of unsaturated carboxylic acid esters, it is presumed that a certain synergistic elTect is obtained from the combination of compounds (i) and (ii) and the unsaturated carboxylic acid, as specified in the present invention.

The polymerization inhibitor in accordance with the present invention shows remarkable stabization effect over prolonged periods, not only when the monomer is kept under low temperatures as in storage, but particularly when the monomer is exposed to elevated temperatures. Furthermore, the stabilization effect is never reduced or impaired when the monomer is in contact with stainless or carbon steel. The polymerization inhibitor of the present invention is also unique in that it exhibits satisfactory stabilization effect inhibitor under such severe conditions as when the monomer alone is exposed to elevated temperatures. Obviously, the inhibitor is extremely useful when added to an esterification reaction system of the unsaturated carboxylic acid. The unsaturated carboxylic acid obtained by the recently noticed method, i.e., preparation of acrylic or methacrylic acid by oxidation of propylene, acrolein, or isobutylene, and methacrolein, contains a number of side products such as aldehyde, acetone, acetic acid, etc., besides the considerably large amount of water. However, the stabilization effect of the subject shows no substantial reduction in the presence of such impuritles.

As described previously, the object of the present invention is essentially achieved by the conjoint use of the aforespecified phosphorus compound (i) and hydroxyl group-containing compound (ii) as the polymerization inhibitor for acrylic or methacrylic acid. Further research disclosed that the stabilization effect of the inhibitor for monomeric acrylic or methacrylic acid is further enhanced by the addition of the third compound (iii) selected from the group consisting of phosphorus acid esters, phosphine derivatives, and benzaldehyde, together with the foregoing compounds (i) and (ii). Furthermore, when compound (i), (ii) and (iii) are used conjointly, the polymerization of the monomer at such places where the monomer is condensed from the gaseous phase, for instance, internal wall surface at the upper part of the distillation column, periphery of the internal wall surface of cooler, or places where the monomer mist stays, can be effectively inhibited.

Now the compounds (i), (ii), and (iii) will be more specifically explained. Examples of thephosphoric acid compounds within the scope of compound (i) include: hypophosphorous acid, phosphorous acid, diphosphorous acid, iso-hypophosphoric acid, ortho-phosphoric acid, pyrophosphoric acid, metaphosphoric acid, and polyphosphoric acid. Whereas, the phosphorus oxide compounds include phosphorus dioxide, phosphorus trioxide, and phosphorus pentoxide. Examples of dihydroxybenzene derivatives as the compound (ii) include hydroquinone, catechol, resorcinol, p-t-butylcatechol, etc. Whereas, the phosphorous acid esters as the compound (iii) include, for example, alkyl phosphites containing l 8 carbon atoms in the alkyl groups, such as dimethyl-, diethyl-, dibuty1-, trimethyl-, triethy1-, tributyl-, and trioctyl-phosphites; and aryl phosphites such as diphenyl-, ditolyl-, dioxylyl-, triphenyl-, tritolyl-, and trixylyl-phosphites. The useful phosphine derivatives within the compound (iii) include, for example, alkylphosphines containing 1 8 carbon atoms in the alkyl groups, such as dimethy1-, diethyl-, dibutyl-, diocty1-, trimethyl-, triethyl-, and trioctylphosphines; and arylphosphines such as diphenyl-, ditolyl-, dixylyl-, triphenyl-, tritolyl-, and trixylyl-phosphines.

ln the addition of the compounds (i) and (ii), or, compounds (i), (ii) and (iii) to the unsaturated carboxylic acid in accordance with the subject method, the order, or manner, of addition is not critical, but any suitable order or means may be employed.

The quantity of the subject polymerization inhibitor [compounds (i) (ii), or (i) (ii) +(iii)] to be added in accordance with the invention is variable over a wide range, depending on the environmental conditions under which the monomer is placed, such as the temperature, and type of the solvent, if the monomer is in the form of solution. Normally satisfactory stabilization can be achieved, by using 0.001 percent by weight, preferably 0.01 3 percent by weight, of the inhibitor based on the monomer, although the use of still greater quantity invites no notable disadvantage.

When the compounds (i) and (ii) are used as the inhibitor, the weight ratio therebetween normally ranges 1.0 0.1 1.0, preferably 1.0 0.3 0.7. Whereas,

when the compounds (i), (ii), and (iii) are used, the weight ratio normally ranges 1.0 0.2 1.0 0.05 0.8, preferably 1.0 0.3 0.8 0.007 0.6.

In accordance with the invention, furthermore, the polymerization inhibitor may be used with conventional stabilizers of unsaturated monomers, to obtain a still greater stabilization elTect.

The invention will be hereinafter explained in further details, with reference to non-limitative working examples.

Examples 1 12 and Controls 1 37 Refined methacrylic or acrylic acid and various polymerization inhibitors of the quantities specified in the following Tables 1 through 4 were each charged in a glass ampule together with a piece of stainless steel, and allowed to stand for 20 hours at C., while the monomer was in contact with the stainless steel piece. The state of the solutions in the glass ampules was observed after 5 and 20 hours of standing, to examine if insoluble polymer was formed. The results are also given in Tables 1 through 4, in which the indication, solidified," signifies that the entire sample system was solidified by polymerization, MAA denotes methacrylic acid, and AA denotes acrylicacid. The sample solutions of the runs within the scope of this invention, which were transparent or contained very minor amounts of precipitate after 20 hours of standing, were thrown into n-hexane, to examine the state of polymer precipitation. According to the degree of polymer precipitation the samples were graded as follows:

A: Substantially no turbidity or very slight turbidity was observed;

B: Only minor amount of polymer precipitation .was observed.

TABLE 1 (Examples 1-12) v v State of State of Ex. Type and Quantity of Inhibitor Mono- Solution Polymer N (wt. td monomer) mer P After I After 1' tion in 5 Hrs. 20 Hrs. n-Hexane 1 Ortho-phosphon'c acid (1.0) MAA Color- Very A +p-cresol 6.5 l light and yellow transbut parent transparent 2 Ortho-phosphoric acid (1.0) do do do A p-cresol (1.) 3 Ortho-phosphoric acid (1.0) do do do A +4,4-thiobis-6-t-butyl-3- methylphenol (0.5) k v 4 Ortho-phosphoric acid 1.0) do do I do p-t-butylcatechol (0.5) 5 Ortho-phosphoric acid 1.0) do do do hydroquinone (1.0) b Ortho-phosphoric acid (1.0 do 10 do A .Zlllflltllli .1 'J'r'hu-dnusmmwu tuztt (r. in (a +resorcinul (1.0) 8 Ortho-phosphoric acid (1.0) do do do A +pyrogallol (1.0) 9 yp osphoric acid (10 do Very minor B l PP fi -lOlill( amount of p-cresol (0.5) precipitate was form d. 10 Phosphorous acid.(1.0) do do Deep yellow A +p-cresol (0.5) and transparent I I Phosphorus Pentoxide (1.0) do (11) vt-r minor n P-" 5 1 .5) amount of precipitate I was formed. 12 Ortho-phosphonc acid (1.0) AA do Deep yellow A p-cresol 1.0) and transparent Table 2 (Controls 1 5) Type and Quantity Control of lnhihitor Mono- State of Solution No. (wt. 71 to monomer) mer After 5 Hrs. After 20 Hrs.

None Solidificd l MAA within Solidified minutes Sodium tripolyphosphatc Precipitatc 2 1.0) hydroquinone dov was formed do.

(0.5) after approx minutes.

Sodium tripolyphosphate Precipitate 3 1.0) hydroquinone do. was formed do.

( L0) after approx.

30 minutes.

Sodium tripolyphosphatc Precipitatc 4 1.0) p-crcsol (0.5) do. was formed do.

after approx.

15 minutes.

Sodium mctuphosphatc 5 l.()) p-crcsol (0.5) do. do. do.

Table 3 (Controls 6 27) yp aniQuu y Control of Inhibitor Mono- State of Solution No. (wt. to monomer) mer After 5 Hrs. After Hrs.

6 Ortho-phosphoric acid MAA solidified Solidificd 7 Polyphosphoric acid do. do. do.

X Phosphorous acid (1.5) do. do. do.

9 Phosphorus pentcyide do. do. do.

l0 p-Crcsol 1.5) MAA Solidificd Solidified ll p-Cresol (1.5) AA do. do.

12 4.4-Thiobis-6-t-butyl- Large amount 3-mcthylphcnol (L5) MAA of prccipido.

rate was formed.

13 p-t-Burylcatcchol (1.5) do. Precipitate Almost was formed solidified l4 Hydroquinone (2.0) do. do. solidified l5 Phenol (L5) do. Solidified do.

l6 Benzylalchol (1.5) do. do. do.

17 Methanol (1.5) do. do. do.

18 Methylene chlorohydrin do. do. do.

19 2.2-Bis(4-hydroxyphenyl) do. do. do.

propane( L5) 20 p-Benzoquinone l.5) do. do. do.

ll Cutcchol l.5) do. Large amount do.

of precipitate was fonned 22 Pyrogullol (1.5) dov do. do.

23 a-Pyridylazo-B-naphthol do. Solidified do,

24 Methylene Blue (1.5) do. do. do.

25 Ethylene thiourcu (L5) do. Preeipitate do.

was formed.

26 Dicthyl hydroxylaminc do. do. do.

27 Phcnothiuzine (l.5) do. do. do.

'l'ahlc 4 (Controls 28 37) Control Type and Quantity of lnhihitor Monomer State of Solution Nu. (wt. '7! to monomer) After 5 Hrs. After Hrs.

28 orlho'phosphorie acid 1.0) 2,2- Precipitatc was Solidificd his(4'-hydroxyphenyl)propane(0.5l MAA formed after 2 hrs.

29 Ortho-phosphoric acid( 1.0) 2.6-

di-t-hutylp eresol(0.5) do. do. do.

30 do. AA do. do.

31 Phosphorous acid 1.0) 2.6-

di-t-butyl-p-cresol (0.5) MAA do. do.

32 Polyphosphoric acid (approx.

cresol (0.5)

33 Phosphorus pentoxide 1.0) 2.6-dit butyl-p-crcsol (0.5) do. do. do.

34 Ortho-phosphoric acid 1.0)

methanol (0.5) do. Solidificd do.

35 Ortho-phosphoric acid (1.0)

ethylcncchlorihydrin (0.5) do. do. do.

36 Ortho-phosphoric acid (1.0)

o o-bisphcnol (0.5) do. do. do.

37 Ortho-phosphoric acid (1.0) Large arrrount Methylene Blue (0.5) do. of precipitate do.

was formed As clearly demonstrated by the results shown in the above four tables. the addition of the compounds (i) and (ii) specified in the present invention to the unsaturated carboxylic acid effectively prevents polymerization of the monomer under elevated temperatures over a prolonged period. In contrast thereto, when the com pound (i) is replaced by polyphosphate such as sodium tripolyphosphate and sodium metaphosphate, which are phosphorus compounds outside the scope of compound (i), no such stabilization effect is attained (refer to Controls 1 5). Again when either of compound (i) or (ii) is used alone, or a hydroxyl group-containing compound outside the scope of compound (ii) (inclusive of conventionally employed stabilizers) is used alone (Controls 6 27), or the hydroxyl groupcontaining compound outside the scope of compound (ii) is used concurrently with the compound (i) (Controls 28 37), substantially no stabilization of the monomer at elevated temperatures can be achieved.

Furthermore, the attempt to prevent polymerization of the ethyl ester of acrylic or methacrylic acid using the inhibitor of the present invention under the same test conditions as above failed to attain the synergistic stabilization effect achieved by the concurrent use of compounds (i) and (ii).

EXAMPLES 13 27 AND CONTROLS 57 The compounds specified in Tables 5 6 below of each specified amount were added to refined methacrylic or acrylic acid. In each run the system was placed in an autoclave in such a manner that the volume of liquid portion (liquid phase portion of the monomer) substantially equalled that of the space portion (gaseous phase portion of the monomer) and that both portions were in contact with stainless steel. The system was allowed to stand for 10 hours, while the liquid phase portion was maintained at C., and the vapor phase portion was externally cooled to 20 30C., under reflux. The state of the solution (liquid phase portion) was observed after 5 and '10 hours of standing, to examine if insoluble polymer was formed. The results are also shown in Tables 5 6, in which the indication, solidified, means that the entire sample was solidified by polymerization. MAA and AA are respectively the abbreviation of methacrylic acid and acrylic acid. The samples from the Examples of this invention which were transparent after 10 hours standing were thrown into n-hexane to examine if polymer would be precipitated. The samples were evaluated for the state of polymer precipitation as below:

A: Substantially no turbidity or very slight turbidity was observed.

B: A large amount of polymer was precipitated. I

Table 5 Exam les 13-27 Table (Examples 13-27) (lnhl'bilor State of (wt.% to monomer) Solution Mono State of No. mer p l Compound Compound Compound After After Precipi- (1) (ii) (iii) 5 1 tation in Hrs. Hrs. n-Hexane Triphcnyl 14 do. do. do. phosphite do. do. do.

Triclhyl 15 do. do. do. phosphilc do. do. do.

Trimethyl 16 do. do. do. phosphilc do. do. do.

Triphenyl 17 do. do. do. phosphinc do. do. do.

Tricthyl 114 do. do. do. phosphine do. do. do.

Benzaldc- 1'9 do. do. do. hyde(0.1 do. do. do.

Phosphor- 20 do. ous acid do. do. do. do. do.

Triphcnyl 21 do. do. do. phosphitc do. do. do.

Orthophos- Calcchol 22 do. phoric (0.2) do. do. do. do.

acid (0.6)

Po1yphosp-Crcsol 23 do. phoric (0.2) do. do. do. do.

acid (0.6)

24 MAA Orlhophos- Pyrogal- Triphcnyl Color- Color- A phoric lo1 phosphitc less less acid (0.6) (0.2) (0.1) and and transtransparent purcn! Phosphorous p-t- Triphenyl 25 do. acid (0.6) butyl phosphinc do. do. do.

calechol (0.1) (0.2)

Orthophosp-crcsol Triphcnyl 26 AA phoric (0.2) phosphite do. do. do.

acid (0.6) (0.1)

Phosphorous 27 do. acid (0.6) do. do. do. do. do.

Table 6 (Controls 38 57) lnhibixor State of State of c M -7Z1 to monomer) Solution Polymer trol "mer Com- Com- Com- Other After After Precipita- No. pound pound pound Com- 5 10 tion in (i) (ii) (iii) pound Hrs. Hrs. n-Hcxanc Orlho- Soliphosdified 3X MAA phoric acid 39 do. Phosphorous do.

acid

40 do. Cresol do.

Hydro- 41 do. quinonc do.

Table 6 (Controls 38-57)- Cont inued c M Inhibitor State of State of [mi mer m n Solution Polymer No. Com- Com- Com- Other After After Precipitapound pound pound Coml0 tion in (i) (ii) (iii) pound Hrs. Hrs. n-Hexanc 42 MAA t-Butyl Solicatcchol dified l .0)

Pyrogal- 43 do. lol do.

Diphenyl 44 do. phosphitc do.

Triphcnyl 45 do. phosphitc do.

Triclhyl 46 do. phosphitc do.

Triphcnyl 47 do. phosphinc do.

Triethyl 48 do. phosphinc do.

Benzaldc- 49 do. hydc do.

Hydroqui- Triphcnyl 50 do. none phosphinc do.

p-Cresol Triphenyl 5! do. (0.6) phosphitc do.

Orthophos- Triphcnyl 52 do. phoric phosphite do.

acid(0.6) (0.4)

Triphenyl 53 do. do. phosphine do.

Hydroqui- Chro- 54 do. none mium do.

(0.6) ace- 55 MAA Ortho- Triphenyl Solidiphosphosphite fled phoric (0.2) acid benzalde- (0.6) hyde(0.2)

Hydro Triphcnyl 56 AA quinone phosphite do.

Benzalde- 57 do. p-Crcsol hydc do.

As clearly demonstrated by the results shown in the above Tables 5 and 6, the addition of the compounds (i), (ii) and (iii) specified in the present invention to the unsaturated carboxylic acid effectively prevented the polymerization of the monomer under elevated temperatures over a prolonged period. No polymer was observed when the surface of the interval wall of the gaseous phase portion was examined in Examples 13 27, whereas polymer formation was observed in Controls 38 57.

We claim:

1. A method of preventing polymerization of an unsaturated carboxylic acid monomer selected from the group consisting of acrylic acid and methacrylic acid. which comprises adding to said acid as the polymerization inhibitor, the combination of (i) at least one phosphorus compound selected from the group consisting of phosphoric acid compounds and phosphorus oxide compounds, and (ii) at least one hydroxyl groupcontaining compound selected from the group consisting of cresols, 4,4-thiobis-6-tert. butyl-3-methylphenol, hydroquinone, catechol. resorcinol. p-t-butylcatechol. and pyrogallol, said polymerization inhibitor being used in the amount of 0.00l 5 percent by weight based on the weight of said unsaturated carboxylic acid.

2. A method of preventing polymerization of an unsaturated carboxylic acid monomer selected from the group consisting of acrylic acid and methacrylic acid, which comprises adding to said acid, as the polymerization inhibitor, the combination of (i) at least one phosphorus compound selected from the group consisting of phosphoric acid compounds and phsphorus oxide compounds, (ii) at least one hydroxyl group-containing compound selected from the group consisting of cresols, 4,4-thiobis-6-tert. butyl-3-methylphenol, hydroquinone, catechol resorcinol, p-t-butylcatechol and pyrogallol, and (iii) at least one compound selected from the group consisting of phosphorous acid esters, alkyl phosphines, the alkyl groups of which contain 1 8 carbon atoms, aryl phosphines and benzaldehyde, said polymerization inhibitor being used in the amount of 0.001 5 percent by weight based on the weight of said unsaturated carboxylic acid.

3. The method of claim 1, wherein said unsaturated carboxylic acid is exposed to elevated temperatures.

4. The method of claim 2, wherein said phosphorous acid esters are alkyl esters, the alkyl groups of which contain 1 8 carbon atoms.

5. The method of claim 2, wherein said unsaturated carboxylic acid is exposed to elevated temperatures.

6. The method of claim 2, wherein said phosphorous acid esters are aryl esters.

7. The method of claim 1 wherein said phosphoric acid compound is selected from the group consisting of hypophosphorous acid, diphosphorous acid, isohypophosphoric acid, ortho-phosphoric acid, pyrophosphoric acid, metaphosphoric acid and polyphosphoric acid.

8. The method of claim 1 wherein said phosphorus oxide compound is selected from phosphorus dioxide, phosphorus trioxide and phosphorus pentoxide.

9. The method of claim 1 wherein the weight ratio of (i) to (ii) ranges from 10:01 1.0.

10. The method of claim 9 wherein the weight ratio of (i) to (ii) is l.0:O.3 0.7.

11. The method of claim 2 wherein the weight ratio of (i) to (ii) to (iii) ranges from l.0:0.2 -1.0:0.05 0.8.

12. The method of claim 11 wherein the weight ratio of (i) to (ii) to (iii) ranges from l.0:O.3 0.8:0.007 0.6. 

2. A method of preventing polymerization of an unsaturated carboxylic acid monomer selected from the group consisting of acrylic acid and methacrylic acid, which comprises adding to said acid, as the polymerization inhibitor, the combination of (i) at least one phosphorus compound selected from the group consisting of phosphoric acid compounds and phsphorus oxide compounds, (ii) at least one hydroxyl group-containing compound selected from the group consisting of cresols, 4,4-thiobis-6-tert. butyl-3-methylphenol, hydroquinone, catechol resorcinol, p-t-butylcatechol and pyrogallol, and (iii) at least one compound selected from the group consisting of phosphorous acid esters, alkyl phosphines, the alkyl groups of which contain 1 - 8 carbon atoms, aryl phosphines and benzaldehyde, said polymerization inhibitor being used in the amount of 0.001 - 5 percent by weight based on the weight of said unsaturateD carboxylic acid.
 3. The method of claim 1, wherein said unsaturated carboxylic acid is exposed to elevated temperatures.
 4. The method of claim 2, wherein said phosphorous acid esters are alkyl esters, the alkyl groups of which contain 1 - 8 carbon atoms.
 5. The method of claim 2, wherein said unsaturated carboxylic acid is exposed to elevated temperatures.
 6. The method of claim 2, wherein said phosphorous acid esters are aryl esters.
 7. The method of claim 1 wherein said phosphoric acid compound is selected from the group consisting of hypophosphorous acid, diphosphorous acid, iso-hypophosphoric acid, ortho-phosphoric acid, pyrophosphoric acid, metaphosphoric acid and polyphosphoric acid.
 8. The method of claim 1 wherein said phosphorus oxide compound is selected from phosphorus dioxide, phosphorus trioxide and phosphorus pentoxide.
 9. The method of claim 1 wherein the weight ratio of (i) to (ii) ranges from 1.0:0.1 - 1.0.
 10. The method of claim 9 wherein the weight ratio of (i) to (ii) is 1.0:0.3 - 0.7.
 11. The method of claim 2 wherein the weight ratio of (i) to (ii) to (iii) ranges from 1.0:0.2 - 1.0:0.05 - 0.8.
 12. The method of claim 11 wherein the weight ratio of (i) to (ii) to (iii) ranges from 1.0:0.3 - 0.8:0.007 - 0.6. 